1. Field of the Invention
The present invention relates to a band-pass surface acoustic wave (SAW) filter including a plurality of SAW resonators and, more particularly, to a SAW filter including a plurality of SAW resonators arranged in a ladder type circuit.
2. Description of the Related Art
A SAW filter having a plurality of SAW resonators arranged so as to define a ladder circuit is disclosed, for example, in Japanese Laid-open Patent Publication Nos. 56-19765 and 5-183380. A SAW filter of this type is now described by referring to FIG. 1.
The SAW filter generally indicated by reference numeral 1 in FIG. 1 includes various electrodes arranged on a rectangular piezoelectric substrate 2 to form a plurality of SAW resonators. In particular, series resonators 3 and 4 each formed of a one-port SAW resonator are connected in series between an input terminal IN and an output terminal OUT. Two parallel arms are located between the input terminal and a reference potential and between the output terminal and the ground potential, respectively, and parallel resonators 5 and 6 each including a one-port SAW resonator are located in the parallel arms.
The resonators 3, 4, 5 and 6 comprise interdigital transducers (IDTs) 3a, 4a, 5a and 6a and grating type reflectors 3b, 3c, 4b, 4c, 5b, 5c, 6b and 6c located on respective opposite sides of a SAW propagating direction.
Each of the IDTs 3a to 6a includes a pair of interdigitated electrodes. One interdigitated electrode of the IDT 3a is electrically connected with the input terminal IN, while the other interdigitated electrode of the IDT 3a is connected with one interdigitated electrode of the IDT 5a and with one interdigitated electrode of the IDT 4a by a connecting electrode 7. The other interdigitated electrode of the IDT 4a is electrically connected with the output terminal OUT and with one interdigitated electrode of the IDT 6a by a connecting electrode 8.
One interdigitated electrode of each of the IDTs 5a and 6a is coupled to a ground potential. Accordingly, in the surface acoustic wave filter 1, the series resonators 3 and 4 are connected in series between the input terminal IN and the output terminal OUT to form the series arm. The two parallel arms are formed between the series arm and the ground potential. In each of these two parallel arms, the parallel resonators 5 and 6 are connected, thus forming a ladder circuit.
The IDTs 3a to 6a and the grating type reflectors 3b, 3c, 4b, 4c, 5b, 5c, 6b, 6c are formed of aluminum or other metal on the piezoelectric substrate 2, together with the connecting electrodes 7 and 8. In the SAW filter 1, the resonant frequency of the SAW resonators 3 and 4 is set to be equal to the antiresonant frequency of the SAW resonators 5 and 6. As a result, the whole SAW filter 1 can exhibit band-pass filter characteristics, which will be described with reference to FIGS. 2 and 3.
FIG. 2 is a plan view schematically showing the electrode structure of each one-port SAW resonator similar to that used to form the resonators 3-6 in FIG. 1. In a SAW resonator 9, an IDT 10 including of a pair of interdigitated electrodes 10a and 10b is located at a central portion of the resonator. The electrodes 10a and 10b are arranged to be interdigitated with each other. Reflectors 11 and 12 are located on opposite sides of the SAW propagation direction in which a surface acoustic wave propagates. The reflectors 11 and 12 comprise a plurality of electrode fingers extending perpendicular to the SAW propagation direction. Both ends of each of the electrode fingers of the reflectors 11, 12 are connected with the ends of other electrode fingers at their respective ends.
When a voltage is applied between the interdigitated electrodes 10a and 10b of the IDT 10, a surface acoustic wave is excited. The excited wave is confined between the reflectors 11 and 12. Hence, a resonator having a high Q can be achieved.
The SAW resonator 9 described above is indicated by a circuit symbol shown in FIG. 3A and has an impedance-frequency characteristic shown in FIG. 3B. As shown in FIG. 3B, the impedance is low near a resonant frequency f.sub.r and is very high at an antiresonant frequency f.sub.a. Therefore, when the SAW resonator 9 and similar SAW resonators are connected in a ladder circuit similar to the SAW resonators 3 to 6 connected as shown in FIG. 1, if the resonant frequency of the series resonators is set to be equal to the antiresonant frequency of the parallel resonators, the input/output impedance can be matched to the characteristic impedance in the vicinity of this frequency. Thus, a passband can be formed.
FIGS. 3C and 3D respectively show a circuit diagram and an impedance-frequency characteristic of the SAW filter 1. Referring to FIG. 3C, when a signal is input to the input terminal IN, the signal component at the resonant frequency f.sub.rs of the series resonators 3 and 4 can be transmitted to the output terminal OUT through the series resonators 3 and 4. In that case, the signal component at the resonant frequency f.sub.rs is not transmitted to the ground since the parallel resonators 5 and 6 have a high impedance at the antiresonant frequency f.sub.ap which is set to be equal to the resonant frequency f.sub.rs of the series resonators 3 and 4. On the other hand, the signal component at the antiresonant frequency f.sub.as of the series resonators 3 and 4 is blocked by the series resonators 3 and 4. The signal component at the resonant frequency f.sub.rp of the parallel resonators 5 is transmitted to the ground potential because of the low impedance at the resonant frequency f.sub.rp. Consequently, the signal component does not reach the output terminal OUT.
Accordingly, the impedance-frequency characteristics shown in FIG. 3D are obtained. As shown in FIG. 3D, attenuation poles are formed at the vicinity of the antiresonant frequency f.sub.as of the series resonators and the resonant frequency f.sub.rp of the parallel resonators. It is noted that a passband PB is defined as a frequency range at which attenuation is within 3 dB (or 6 dB) with respect to the center frequency of the filter or a minimum insertion loss. A stopband SB is also defined as a frequency range at which attenuation is within 20 dB (or 40 dB) with respect to the center frequency of the filter or a minimum insertion loss.
According to the aforementioned description of the conventional device, such a SAW filter is arranged to have a band-pass filter characteristic which provides low insertion loss and large attenuation in a stopband at the vicinity of the passband.
The band-pass filter characteristic of the aforementioned SAW filter 1 may be sufficient to provide the filter characteristics for a band-pass filter which were required several years ago. However, in recent years, the interval between the transmission frequency and the reception frequency has become very close in a communications device such as a cellular mobile telephone to enhance the efficiency of utilization of electromagnetic waves. Therefore, the SAW filter 1 does not always provide sufficient selectivity. Thus, the inventors have determined that there is a need for a band-pass filter having a steeper filtering characteristic curve or frequency response between a passband and a stopband as compared to presently available devices.
In order to increase the sharpness or steepness of the frequency response (attenuating characteristic) between the passband and stopband, it is conventionally thought that a general method in which the number of resonators is increased to increase the total number of the resonator stages should be used. A resonator stage comprises a pair of series and parallel SAW resonators. However, when the number of stages is increased, the electrode resistance is undesirably increased in proportion to the number of resonators. This leads to a deterioration of the insertion loss for the filter. Furthermore, the process of arranging the increased number of electrodes on the piezoelectric substrate becomes much more complicated, which in turn renders the manufacturing and assembling process more complex and also increases the size of the piezoelectric substrate and overall filter. In this way, limitations are imposed on the filter design and manufacturing method which includes increasing the number of stages. For the foregoing reasons, the inventors have determined that there is a demand for a SAW filter which achieves a steeper frequency response at an interface between a passband and a stopband without increasing the number of the stages in the SAW filter.